Non-aqueous secondary batteries (hereinafter may simply referred to as “secondary batteries”) such as lithium ion secondary batteries are small and light, high in energy density, and capable of repeated cycles of charge and discharge. For such characteristics, secondary batteries are used in a wide variety of applications. A secondary battery typically includes battery members such as a positive electrode, a negative electrode, and a separator for separating the positive and negative electrodes from each other to prevent a short-circuit between them.
In recent years, secondary battery members have been used that have a porous membrane layer for increasing heat resistance and strength, an adhesive layer for bonding together battery members and other like layers (hereinafter such layers may be collectively referred to as “functional layers”). Specifically, electrodes that include a functional layer formed on an electrode substrate formed of an electrode mixed material layer provided on a current collector, and separators that include a functional layer formed on a separator substrate have been used as battery members.
Extensive improvements have been made on functional layers in an aim to achieve higher performance of secondary batteries that have battery members having such functional layers (see, e.g., PTL 1).
Specifically, for example, PTL 1 proposes using as a binder a carboxyl-modified diene polymer containing an aliphatic conjugated diene monomer unit at an amount of 5 to 85% by mass in porous membrane layers that contain fillers and binders to increase the binding strength of the porous membrane layer while limiting binder aggregation.